There are different ways to measure a flow rate of fluids (liquids and gases).
One way is to include some kind of moving element in direct contact with the fluid that serves to measure the flow rate.
Another way is to measure the flow rate without such moving element, for example, is by using Coriolis, magnetic, ultrasonic, vortex, and thermal technologies.
An example mass flow meter which operates according to the Coriolis principle is disclosed in U.S. Pat. No. 6,041,665.